| BREEDING,CULTIVARS,ROOTSTOCKS, AND GERMPLASM RESOURCES HORTSCIENCE 53(6):777–781. 2018. https://doi.org/10.21273/HORTSCI12748-18 racesA,B,C,andD.B2Carosefrom crosses of PI 195342, ‘Bridgeton’ and US 1068, a sibling of ‘Bridgeton’ (Thomas and Inheritance of Resistance in Lima Bean Fisher, 1980). Phytophthora phaseoli The lima bean improvement program to , the Causal conducted by USDA-ARS, Beltsville, be- tween 1949 and 1989, resulted in significant Agent of Downy Mildew of Lima Bean improvement in quality, yield, and most importantly, resistance to the four prevalent 1 Luisa Santamaria races of P. phaseoli (Evans et al., 2007; Kee Department of Botany and Plant Pathology, North Willamette Research & et al., 1997). From this program, more than Extension Center, Oregon State University, Aurora, OR 97002 30 cultivars and germplasm releases were made available to plant breeders, seed com- Emmalea G. Ernest, Nancy F. Gregory, and Thomas A. Evans panies, and producers (Stavely, 1991). Department of Plant and Soil Sciences, University of Delaware, Newark, DE For 20 years, no new races of P. phaseoli 19716 were detected in the Mid-Atlantic region (MAR). However, in 1995, a new race of P. Additional index words. Phaseolus lunatus, oomycete, screening for disease resistance, phaseoli, designated race E, was detected in germplasm collection, single dominant gene Delaware and became more widely distrib- Abstract Phytophthora phaseoli uted than race D (Evans et al., 2002). In 2000, . The oomycete is one of the most threatening pathogens of an epidemic incited by race E resulted in an lima bean (Phaseolus lunatus) in the humid Mid-Atlantic United States. In the last 60 P. phaseoli estimated loss of 40% of Delaware lima bean years, has evolved to overcome genetic resistance in the host and several production, equal to a farm gate value loss physiological races have been identified during the last 6 decades. Six physiological races of $3,000,000 (Scuse and Feurer 2003). A, B, C, D, E, and F have been identified over the years. Only race F has been detected in Then, race F of P. phaseoli was first detected the field over the past decade. Identifying and characterizing sources of resistance to this in 2000 (Evans et al., 2005) and was the most pathogen and determining the nature of resistance were the main objectives. Eight prevalent race by 2004. Because of the commercial cultivars and 35 germplasm accessions of P. lunatus were evaluated for their emergence of the E and F races, it is impor- reaction to races E and F. Four commercial cultivars and four accessions with resistance to race E, and two cultivars and four accessions with resistance to race F were identified. tant to evaluate the reaction of the cultivars None of the germplasm evaluated were resistant to both races. Five populations of F2 most commonly grown in Delaware to these plants and a recombinant inbred line (RIL) population were produced and inoculated to new races and identify possible sources of investigate the inheritance of resistance to races E and F. Resistance to races E and F was resistance. There are currently no cultivars determined to be conferred by single, independent, dominant genes. with resistance to both races E and F of P. phaseoli, resistance to race F is needed in horticulturally acceptable germplasm for use Downy mildew, incited by P. phaseoli races A, B, C, D, E, and F have been reported in the humid MAR. Thaxt., became a major threat to lima bean by Evans et al. (2007). In 2004, a lima bean breeding program (P. lunatus L.) production in the humid The USDA downy mildew resistance was re-initiated at the University of Delaware eastern United States in the 1940s. The U.S. breeding program began in 1948 and in to develop new cultivars for the MAR, in- Department of Agriculture (USDA) began 1958, released ‘Thaxter’, its first cultivar cluding cultivars with resistance to races E breeding lima bean for resistance to downy with downy mildew resistance (Wester and and F of P. phaseoli. This study was un- mildew in 1948 in collaboration with the Cetas, 1959). The resistance in ‘Thaxter’ was dertaken to support the disease resistance Long Island Vegetable Research Farm at conferred by a single dominant gene derived breeding goals of the program. The first Riverhead, NY (Wester and Cetas, 1959). from the P. lunatus landrace PI 164155. A objective of was to evaluate the reactions of At that time, no races or variants of the new race of P. phaseoli, which overcame the commercial cultivars to races E and F of P. pathogen were known. A physiological race, resistance in ‘Thaxter’, was detected in 1958 phaseoli in the field. The second objective hereafter referred to as race, is a subdivision and was designated race B (Wester and Cetas, was to identify sources of resistance to those of a pathogen species, particularly fungi, 1959). The landrace PI 189403 was found to races from diverse germplasm, including distinguished from other members of the be resistant to races A and B (Wester and cultivars, germplasm releases, and landraces. species by specialization for pathogenicity Cetas, 1959) and was used as the resistant The third objective was to determine the to different cultivars of a host (Kirk et al., parent in the development of ‘Dover’, which pattern of inheritance of resistance to these 2001). The use of differential cultivars has was released by the USDA in 1970 (Stavely, two races to facilitate incorporation of re- been the principal method to distinguish 1991). A new race of downy mildew, desig- sistance genes into commercially acceptable emerging races of P. phaseoli. Differential nated C, which overcame the resistance in small, flat-seeded (baby), and large, plump- cultivars used to distinguish physiological Dover, was identified in 1969 when the seeded (Fordhook), lima bean cultivars for cultivar was still in trial (Wester, 1970). production in the MAR. The PI 195342 was found to be resistant to Received for publication 5 Jan. 2018. Accepted for the new race C as well as races A and B and Materials and Methods publication 6 Apr. 2018. were used in the development of the germ- This research was supported by USDA/AMS plasm C-171, which was released in 1973 and Maintenance of physiological races of P. Award Agreement 12-25-G-0543 and USDA/ was resistant to races A, B, and C (Thomas phaseoli. Isolates of P. phaseoli character- AMS Award Agreement 2-25-B-1059. and Fisher, 1979). In 1975, a fourth race of ized as races E and F were maintained in pure WethankPaulLaytonforhisworktolocateandacquire downy mildew, designated D, was identified. cultures for long-term storage at 20 °Cin seed from the wild population of P. polystachios. PI 195342, used since 1972 as a source of screw-cap test tubes containing sterile water Mention of a trademark, proprietary product, or resistance to races A, B, and C, was sus- and corn (Zea mays L.) and/or cucumber vendor does not constitute a guarantee or warranty ceptible to race D (Thomas and Blount, of the product by the authors and does not imply its (Cucumis sativus L.) seeds as substrates. Iso- approval to the exclusion of other products or 1976). A germplasm release designated lates of P. phaseoli races E and F were vendors that may also be suitable. B2C (PI 549515), made jointly by USDA- reactivated by transferring to the semiselective 1Corresponding author. E-mail: luisa.santamaria@ ARS and the Delaware Agricultural Experi- media lima bean agar (Calvert et al., 1960) or oregonstate.edu. ment Station in 1976, had resistance to modified pea agar (Dance et al., 1975). After HORTSCIENCE VOL. 53(6) JUNE 2018 777 transfer, the cultures were maintained in an 2003 to 2010. Nineteen accessions were from to races E and F of P. phaseoli was started. The incubator set at 20 °C. the USDA-National Plant Germplasm Sys- cultivars used in the study were Cypress, Inoculum production and hypocotyl tem and 16 from the International Center for Dover Tucker, Maffei 15, Jackson Wonder, screening of germplasm. Inoculum of P. Tropical Agriculture. The remaining eight and germplasm release B2C. Parents for the phaseoli was produced and maintained on were cultivars obtained from commercial crosses were chosen based on their disease hypocotyls of the susceptible lima bean culti- seed suppliers. A species native to eastern reaction in previous disease screening: ‘Cy- var Concentrated Fordhook. Untreated seeds North America, Phaseolus polystachios (L.) press’ and ‘Dover Tucker’ are resistant to race were rinsed in sterile distilled water to remove BSP, was also screened to determine its E but susceptible to race F, whereas ‘Maffei any surface contaminants and to begin the reaction to races E and F in the field (four 15’ and B2C are susceptible to race E and imbibition process. The seeds were placed in plants for each race), and in a dew chamber resistant to race F (Table 1). ‘Jackson Wonder’ 10-cm plastic pots with a commercial plug and (five plants for each race) during 2010. Seed is susceptible to both races. Crosses were seedling mix, five to six seeds per pot, placed used for screening was collected from a wild made between ‘Jackson Wonder’ and the close together. The pots were placed on heat population in Maryland and subsequently resistant parents in the greenhouse. The F1 mats set to 26 °C, on greenhouse benches. submitted to the USDA-NPGS collection, plants from these crosses were planted in the Four days after seeding when hypocotyls accession number W636674. The species P. greenhouseandtheseedfromeachF1 plant began to emerge, some potting mix was gently polystachios, which was not previously iden- was harvested and maintained separately dur- removed by hand from the pots to reveal more tified as a host of P.